The Marian reforms were alleged changes to the composition and operation of the Roman army during the late Roman republic usually attributed to Gaius Marius (a general who was consul in 107, 104–100, and 86 BC). The most important of those alleged changes concerned the altering of the socio-economic background of the soldiery. Other changes were supposed to have included the introduction of the cohort; the institution of a single form of heavy infantry with uniform equipment; the universal adoption of the eagle standard; and the abolition of the citizen cavalry. It was commonly believed that Marius changed the soldiers' socio-economic background by allowing citizens without property to join the Roman army, a process called "proletarianisation". This proletarianisation was then supposed to have created a semi-professional class of soldiers motivated by land grants which in turn became clients of their generals, who then used them to overthrow the republic.
Belief in a comprehensive scheme of reforms under Marius emerged in 1840s German scholarship, which posited that any changes in the Roman army between the times of Polybius and Marius were attributable to a single reform event. This belief was spread relatively uncritically and was accepted as largely proven by the 1850s and through much of the 20th century. There is, however, little ancient evidence for any permanent or significant change to recruitment practice in Marius's time. The occurrence of such a comprehensive reform led by Marius is no longer widely accepted by specialists; 21st-century scholars have called the reforms a "construct of modern scholarship".
Other reforms to the army's operations and equipment, said to have been done by Marius, are also largely rejected by scholars. Few of them have any basis in the ancient and archaeological evidence. Others are wrongly dated or misattributed. Changes in the Roman army of the late republic occurred both later (during the Social War and following civil wars rather than at the end of the 2nd century BC) and emerged from circumstance rather than a reformist Marian vision.
Background
The Roman army traditionally found its manpower by conscription from the top five census classes. Those classes were assigned in decreasing order of wealth and allotted citizens to a corresponding century in the . These citizens were called . Citizens who owned less wealth than that required for bottom of the fifth census class were called () or . These least-wealthy citizens were grouped into a single century which voted after all the others. Under the this scheme, the were exempt from conscription except when an emergency, called a , was declared; under such circumstances, the poorest were levied as well. The first documented instance of the being called up was some time in the fourth century; they first received arms at state expense in 281 BC, probably related to the start of the Pyrrhic War.
For much of the 20th century, historians held that the property qualification separating the five classes and the was reduced over the course of the second century to a nugatory level due to a shortage of manpower. The basis for that belief, however, was merely three undated Roman figures for the amount of property required to serve which would serve as evidence for reductions only if forced into a descending order. Many scholars have also now abandoned the notion that Italy suffered in the second century BC any deficit of manpower which would have driven such putative reductions.
Attributed reforms
In traditional modern historiography, some or all of the following reforms are attributed to Marius. They are, however, variably dated. Many modern sources date them to his first consulship, during the Jugurthine War against Jugurtha of Numidia, in 107 BC. However, it is also possible that other far-reaching actions, especially in opening army recruitment, were undertaken during Marius's repeated consulships from 104 to 100 BC during which Rome faced the serious threat of Germanic invasion.
Ancient attributions
Beyond the attribution to Marius of setting the precedent for recruiting the poor, made by the historian Valerius Maximus in the early 1st century AD, only two reforms (distinguished from mere actions taken by Marius) are attributed, in sources postdating his career by hundreds of years, to Marius directly: a redesign of the and sole use of the eagle as the legionary standard.